This invention relates to the field of seismic geophysical exploration and more particularly to an improved source of acoustic waves for seismic exploration.
Seismic geophysical exploration in general involves the generation of acoustic waves at or near the earth's surface and the recording at the earth's surface of acoustic waves reflected from subsurface interfaces. Originally, the acoustic wave source was simply a dynamite charge in a shallow shot hole. A newer alternative to the dynamite or impulsive source is the mechanical or hydraulic vibrator which is used to generate a swept frequency signal at the earth's surface. When such swept frequency vibratory signals are used, the reflected signals detected at the earth's surface are correlated with a pilot signal which corresponds to the signal initially generated. After the correlation procedure, such received signals appear to be essentially the same as the reflected signals obtained when an impulsive source is used.
A number of U.S. patents, thought to be relevant to the present application, include U.S. Pat. No. 2,064,451, issued to Voorhees; U.S. Pat. No. 2,320,248, issued to Shimeck; U.S. Pat. No. 2,377,903, issued to Rieber; and U.S. Pat. No. 2,779,428, issued to Silverman.
The first three of these patents deal with the use of multiple impulsive sources in efforts to emphasize certain seismic frequencies, and, in some cases, to provide directional responses. For example, the Voorhees patent involves the use of a number of individual charges spaced vertically apart in a shot hole and fired simultaneously. As stated in that patent, the result is a sustained seismic wave having a single or predominating wavelength and frequency and traveling in a generally downward direction. The resulting wave is in essence a monofrequency burst having a considerable overall length in time and space which reduces its effectiveness at defining thin strata.
The Shimeck patent involves the use of multiple charges fired at essentially the same point in a shot hole but time-spaced. The resulting downward traveling wave is essentially identical to that of Voorhees except that the emphasized frequency is a function of the time spacing of the shot firings instead of the physical spacing in the shot hole. Again Shimeck produces in essence a long monofrequency burst and he suggests detecting it with finely tuned amplifiers and requires integration of the detected signals.
The Reiber patent differs from Shimeck in having the individual charges spaced laterally from each other to some extent. As a result, the effective wavetrain frequency varies with the angle of propagation from the charge array. In this way, the array is said to provide directional response. Again, the time-spaced firings generate a fairly long monofrequency wavetrain and tuned amplifiers are suggested for detecting the return signals.
Each of these three patents, therefore, involves the use of impulsive sources to emphasize a single frequency of seismic energy but does so at the cost of generating a monofrequency wave train of long time duration. As a result, these types of signals cannot be used to distinguish interfaces which are close together, such as the top and bottom of a thin hydrocarbon-bearing sand. In other words, seismograms produced from the signals used in these patents would display poor resolution.
The Silverman patent referenced above involves the use of multiple charges fired at a nonrepetitive time-spacing sequence. The received signals are in effect correlated with a time sequence representing the firing schedule of the individual charges. In this way, the composite long time interval wavelet is compressed into a short spike in the same method of operation as swept frequency systems which employ vibrators. The Silverman system, therefore, does not lose resolution by using multiple time-spaced charges but does require the use of correlation processing equipment.
Thus, it is seen that it has been known that particular frequencies can be emphasized by multiple firing of individual impulsive sources, and it has been known that impulsive sources can be used in fairly high resolution systems using correlation techniques, but a method of firing multiple charges to give selective frequency emphasis has not been described which could be employed in high resolution seismic prospecting without the use of a correlation process.
Accordingly, an object of the present invention is to provide a method for high resolution seismic prospecting using multiple impulsive sources without requiring correlation of received signals.
According to the present invention, a seismic initiation is generated by means of a closely spaced time sequence of simple source pulses having amplitudes and time spacing selected by initially measuring the response of the earth transmission system to a single pulse. Resulting recorded signals reflected from subsurface interfaces contain a relatively broad and flat spectrum of frequencies.